With the increasing miniaturization of a semiconductor integrated circuit element in recent years, exposing light with a shorter wavelength has been used in a lithographic step. At present, the use of a KrF excimer laser (with a wavelength of 248 nm) or the like is becoming prevalent.
As the wavelength of exposing light becomes shorter, the reflectivity of light reflected from a substrate after exposing a resist film becomes higher so that the reflected light from the substrate is more likely to cause variations in the size of a resist pattern. The variations in the size of the resist pattern are produced as follows. After exposing the resist film, the light is reflected from the substrate underlying the resist film and incident again on the resist film to re-expose the resist film including a portion which should not be exposed.
To prevent the reflected light from being incident on the resist film, there has recently been proposed a process wherein an organic anti-reflection coating (hereinafter simply referred to as ARC in tables and drawings) is formed under the resist film. The process is primarily used in the manufacturing of a semiconductor element in a high-performance device with design rules whereby a gate width is 0.25 .mu.m or less.
Referring to FIGS. 1(a) to 1(e), a method of forming a resist pattern by using an organic bottom anti-reflective coating will be described.
First, as shown in FIG. (1a), an organic bottom anti-reflective coating 12 is deposited on an underlying film 11 (such as a polysilicon film, a silicon oxide film, or a tungsten silicide film) to have a flat surface. Then, as shown in FIG. 1(b), a resist film 13 composed of a positive resist is deposited on the organic bottom anti-reflective coating 12.
Next, as shown in FIG. 1(c), selective exposure of the resist film 13 is performed by using a mask 14. Subsequently, developing treatment is performed with respect to the exposed resist film 13 to remove the exposed portion thereof, thereby forming a resist pattern 15 shown in FIG. 1(d).
Next, as shown in FIG. 1(e), dry etching is performed with respect to the organic bottom anti-reflective coating 12 masked with the resist pattern 15, thereby removing the portion of the organic bottom anti-reflective coating 12 uncovered with the resist pattern 15.
However, the pattern formation method using the organic bottom anti-reflective coating 12 as described above presents problems during dry etching performed with respect to the organic bottom anti-reflective coating 12, which are easily produced variations in the size of the organic bottom anti-reflective coating 12, low selectivity of the organic bottom anti-reflective coating 12 to the underlying film 11, and an increased number of particles generated in a reaction chamber in which etching is performed.
As for the problem of easily produced variations in the size of the organic bottom anti-reflective coating 12, the cause thereof may be simultaneous etching of the resist pattern 15 with the etching of the organic bottom anti-reflective coating 12 since the resist pattern 15 is made of a carbon-based material, similarly to the organic bottom anti-reflective coating 12.
As for the problem of an increased number of particles generated in the reaction chamber, the cause thereof may be particles generated from the organic bottom anti-reflective coating 12 during the etching of the organic bottom anti-reflective coating 12.
To solve each of the above problems, there has been proposed the use of HBr/O.sub.2 -based gas and N.sub.2 /O.sub.2 -based gas as etching gas for the organic bottom anti-reflective coating 12.
On the other hand, it has recently been reported that the use of Cl.sub.2 /O.sub.2 -based gas as etching gas for the organic bottom anti-reflective coating 12 improves size controllability and maximizes the selectivity to a polysilicon film as the underlying film (NEC: Nishizawa et al., the 57th Applied Physics Scientific Lecture Meeting (Autumn 1996), No.2 p.483,7a-T-1). It has also been reported that the use of CO/N.sub.2 /O.sub.2 -based gas as etching gas for the organic bottom anti-reflective coating 12 improves the selectivity to the resist film (LGSemicon: Jeon et al., the 57th Applied Physics Scientific Lecture Meeting (Autumn 1996), No.2 p.522, 8a-T-7).
(Problem Caused by Etching Using Cl.sub.2 /O.sub.2 -Based Gas)
The present inventors performed dry etching with respect to the organic bottom anti-reflective coating 12 by using Cl.sub.2 /O.sub.2 -based gas and encountered another problem, which will be described below.
A description will be given to a method of dry etching performed with respect to an organic bottom anti-reflective coating by using Cl.sub.2 /O.sub.2 -based gas. Although a dry-etching apparatus can be selected from various etching apparatus, the description will be given to the case where dry etching was performed by using a first etching apparatus shown in FIG. 2.
The first etching apparatus comprises a grounded chamber 21 having an inner wall covered with an insulator such as ceramic, alumina, or quartz.
An inner circumferential wall of the chamber 21 is provided with a first lateral electrode 22A, a second lateral electrode 22B, and a third lateral electrode 22C, which are equally spaced therearound. The first, second, and third lateral electrodes 22A, 22B, and 22C receive respective supplies of high-frequency power of 54.24 MHz from a first high-frequency power source 23A, a second high-frequency power source 23B, and a third high-frequency power source 23C via a matching circuit not shown. The supplies of high-frequency power are equal in discharge power but progressively shifted in phase by approximately 120.degree.. Specifically, the phase of high-frequency power supplied to the second lateral electrode 22B is 120.degree. leading the phase of high-frequency power supplied to the first lateral electrode 22A and the phase of high-frequency power supplied to the third lateral electrode 22C is 120.degree. lagging behind the phase of high-frequency power supplied to the first lateral electrode 22A. It is to be noted that a phase shifter not shown produces a 120.degree. phase shift between the supplies of high-frequency power from each adjacent two of the first to third high-frequency power sources 23A to 23C.
An earth electrode 24 is disposed on the inner bottom portion of the chamber 21. A sample stage 25 serving as a lower electrode for holding a substrate is provided on the earth electrode 24. A bias high-frequency voltage is applied from a fourth high-frequency power source 26 to the sample stage 25.
The chamber 21 is provided with a gas inlet for introducing etching gas into the chamber 21 via a mass flow controller and with a turbo pump for adjusting pressure in the chamber 21 to be about 0.1 to 10 Pa, though they are not shown in the drawings.
A description will be given below to a method of dry etching performed with respect to the organic bottom anti-reflective coating by using the first etching apparatus with reference to FIGS. 4(a) and 4(b).
First, as shown in FIG. 4(a), a silicon oxide film 42 is thermally grown on a silicon wafer 41, followed by a polysilicon film 43 deposited as an underlying film on the thermally grown silicon oxide film 42. Thereafter, an organic bottom anti-reflective coating 44 with a film thickness of 150 nm and a resist film are deposited sequentially on the polysilicon film 43. Then, etching is performed with respect to the resist film to form a resist pattern 45 composed of the resist film.
Next, as shown in FIG. 4(b), dry etching is performed with respect to the organic bottom anti-reflective coating 44 masked with the resist pattern 45. Conditions for the dry-etching process are as shown in Table 1.
In Table 1, LEP (Lissajous Electron Plasma) denotes the frequencies and power of high-frequency power for plasma generation supplied from the first to third high-frequency power sources 23A, 23B, and 23C and RF (Radio Frequency) denotes the frequency and power of the bias high-frequency power supplied from the fourth high-frequency power source 26. The denotations of LEF and RF remain the same in the following description and tables showing the conditions for the etching process.
TABLE 1 CONDITIONS FOR ETCHING PROCESS Cl.sub.2 /O.sub.2 20/20 (sccm) LEP/RF 54.24/13.56 (MHz) 3 .times. 100/70 (W) PRESSURE 5 (mTorr) TEMPERATURE OF -5 (.degree. C.) LOWER ELECTRODE
The results of etching are as shown in Table 2. After etching, the organic bottom anti-reflective coating 44 exhibited vertical profiles, an excellent size-varying property, and high selectivity to the polysilicon film 43. In Table 2, "open" denotes a wiring pattern area in which a space forming a line-and-space pattern has a width of about 1 .mu.m or more and "dense" denotes a wiring pattern area in which a space forming a line-and-space pattern has a width of about 0.3 .mu.m. The denotations of "open" and "dense" remain the same in the following description and tables showing the conditions for the etching process.
TABLE 2 ETCHING PROPERTIES ARC ETCH RATE 1600 (.ANG./min) UNIFORMITY .+-.2.0 (%) SIZE VARIATION -0.02 .mu.m (open) -0.05 .mu.m (dense) SELECTIVITY TO .infin. POLYSILICON FILM ROUGHNESS OF PRESENT POLYSILICON FILM NUMBER OF SMALL GENERATED PARTICLES
In a surface region of the silicon wafer 41, especially the region where the resist pattern 45 has an aperture small in area (dense area), the polysilicon film 43 underlying the organic bottom anti-reflective coating 44 is also etched, so that surface roughness is observed at the polysilicon film 43. When etching performed with respect to the organic bottom anti-reflective coating 44 has thus caused surface roughness at the polysilicon film 43, roughness remains on the polysilicon film 43 formed into a pattern, which presents a serious problem to practical applications.
(Problem Caused by Etching Using N.sub.2 /O.sub.2 -Based Gas)
The present inventors also performed dry etching with respect to the organic bottom anti-reflective coating by using N.sub.2 /O.sub.2 -based gas and encountered still another problem, which will be described below. A description will be given to a method of etching performed with respect to the organic bottom anti-reflective coating by using N.sub.2 /O.sub.2 -based gas. Although a dry-etching apparatus can be selected from various etching apparatus, the description will be given to the case where dry etching was performed by using a second etching apparatus shown in FIG. 3.
The second etching apparatus comprises a grounded chamber 31 having an inner wall covered with an insulator such as ceramic, alumina, or quartz. In the chamber 31, an inductively coupled coil 32 is disposed in the upper part thereof, while a sample stage 33 is disposed on an earth electrode 34 in the lower part thereof. One end of the inductively coupled coil 32 receives high-frequency power of 13.56 MHz applied thereto from a first high-frequency power supply 35 via a matching circuit not shown. The sample stage 33 receives high-frequency power of 13.56 MHz applied thereto from a second high-frequency power supply 36. The other end of the inductively coupled coil 32 is connected to the sidewall of the chamber 31 and thereby grounded. The chamber 31 is provided with a gas inlet for introducing etching gas into the chamber 31 via a mass flow controller and with a turbo pump for adjusting pressure in the chamber 31 to about 0.1 to 10 Pa, though they are not shown in the drawings.
A description will be given below to a method of dry etching performed with respect to the organic bottom anti-reflective coating by using the second etching apparatus with reference to FIGS. 4(a) and 4(b).
First, as shown in FIG. 4(a), a silicon oxide film 42 is thermally grown on a silicon wafer 41, followed by a polysilicon film 43 deposited as an underlying film on the thermally grown silicon oxide film 42. Thereafter, an organic bottom anti-reflective coating 44 with a film thickness of 150 nm and a resist film are deposited sequentially on the polysilicon film 43. Then, etching is performed with respect to the resist film to form a resist pattern 45 composed of the resist film.
Next, as shown in FIG. 4(b), dry etching is performed with respect to the organic bottom anti-reflective coating 44 masked with the resist pattern 45. Conditions for the dry-etching process are as shown in Table 3. In Table 3, ICP (Inductively Coupled Plasma) denotes the frequency and power of high-frequency power for plasma generation supplied from the first high-frequency power supply 35. The denotation of ICP remains the same in the following description and tables showing the conditions for the etching process. The results of etching are as shown in Table 4. After etching, the organic bottom anti-reflective coating 44 exhibited vertical profiles, an excellent size-varying property, and high selectivity to the polysilicon film 43, as can be understood from Table 4.
TABLE 3 CONDITIONS FOR ETCHING PROCESS N.sub.2 /O.sub.2 20/20 (sccm) ICP/RF 13.56 (MHz) 300/70 (W) PRESSURE 5 (mTorr) TEMPERATURE OF 10 (.degree. C.) LOWER ELECTRODE
TABLE 4 ETCHING PROPERTIES ARC ETCH RATE 1600 (.ANG./min) UNIFORMITY .+-.2.0 (%) SIZE VARIATION -0.04 .mu.m (open) -0.06 .mu.m (dense) SELECTIVITY TO .infin. POLYSILICON FILM FOREIGN RESIDUE PRESENT
After dry etching, the silicon wafer 41 as a target substrate was visually inspected in the dark field under a microscope and a large number of (about 100 residues per 3 mm.sup.2) were observed. Such foreign residues resulting from etching performed with respect to the organic bottom anti-reflective coating 44 present a serious problem to practical applications, since the foreign residues are transferred to the polysilicon film 43 while it is etched in the subsequent step.
In view of the foregoing, it is therefore a first object of the present invention to pattern an organic bottom anti-reflective coating by dry etching such that the patterned organic bottom anti-reflective coating exhibits vertical profiles, an excellent size-varying property, and high selectivity to the underlying film, while no surface roughness is observed at the underlying film. A second object of the present invention is to pattern an organic bottom anti-reflective coating by dry etching such that the patterned organic bottom anti-reflective coating exhibits vertical profiles, an excellent size-varying property, and high selectivity to the underlying film, while no foreign residue is produced.